My 11 year old daughter shoots an 18# recurve and I have her shooting 28" Easton Axis 600's with 150 grains up front. She's drawing about 23 inches. Axis shafting is not cheap, but they won't break for you. Whatever you choose, bright wrap and fletching will help with arrow loss.
My main point is if you go with carbon, leave them long (28" is ok) and you may have to use heavier point, but that's ok, since you can drop point weight or cut them down slightly as the grow into bigger bow. The narrow carbon shafting will put the arrow closer to center shot, therefore that will help "stiffen" the spine in essence, with your light setup. Good luck.